This project will evaluate the molecular mechanisms by which 1,25 dihydroxyvitamin D (1,25(OH)2D) regulates intestinal calcium transport. The approach will be study calcium transport across purified plasma membranes from the luminal (brush border) and antiluminal (basolateral) ends of the intestinal epithelial cell. The goal is to test a model of 1,25(OH)2D regulated intestinal calcium transport which postulates that 1,25(OH)2D stimulates calcium movement into the cell across the brush border membrane by a mechanism not dependent on protein synthesis, whereas the subsequent stimulation of calcium removal from the cell at the basolateral membrane requires protein synthesis. The project will focus on two calcium binding proteins thought to play a role in calcium transport: the ubiquitous multipurpose calcium binding protein calmodulin and the vitamin D inducible calcium binding protein CaBP. Unlike CaBP, calmodulin production is not increased by 1,25(OH)2D although 1,25(OH)2D has profound effects on the distribution of calmodulin within the cell; within hours after 1,25(OH)2D administration calmodulin levels in the brush border and basolateral membranes are increased. Therefore, calmodulin may mediate the ability of 1,25(OH)2D to increase calcium permeability at the brush border either by a direct effect on the calcium channel or an indirect effect by stimulating phospholipid turnover (via phospholipase A2) or membrane protein phosphorylation (via protein kinase). None of these effects involves protein synthesis. In contrast, although calmodulin may mediate some of the effects of 1,25(OH)2D on calcium flux across the basolateral membrane by stimulating CaATPase, CaBP may also be involved at this step explaining the correlation between CaBP production and calcium removal from the cell in vivo. The project will assess these possibilities directly. If successful this project will not only expand our understanding of the mechanism of action of a very important hormone regulating calcium homeostasis, but may also produce important information linking dietary calcium to intestinal calcium absorption. Such information would provide additional rationale for calcium supplementation of individuals with calcium malabsorption including most elderly and postmenopausal subjects.